Grafana “pd -> current storage used” panel can display storage capacity separately

## Feature Request

**Is your feature request related to a problem? Please describe:**
The grafana pd -> current storage used panel cannot distinguish between tikv or tiflash caused some confusion。

**Describe the feature you'd like:**
Display storage usage for tikv and tiflash separately。

**Describe alternatives you've considered:**
Use the tikv_store_size_bytesikv_store_size_bytes metric of tikv and the tiflash_system_current_metric_StoreSizeCapacity metric of tiflash instead of the pd_cluster_status metric of pd